1. Field of the Invention
This invention relates to optical systems and, in particular, to optical systems used for making selective comparative inspections.
2. Description of the Prior Art
To achieve acceptable circuit yields and high reliability in integrated circuits and thin film circuits presently requires the performance of a visual inspection to verify the integrity of both mask and metallization patterns. Recent design trends toward higher levels of integration in both integrated and film circuits have increased the need for pattern verification. However, the resulting pattern complexity of integrated and film circuits has made visual inspection of both masks and metallization patterns more difficult and more expensive. Typically, an inspector must scan a pattern, detect potentially harmful defects in the mask or metallization patterns and decide which of the detected defects may indeed be critical to circuit performance. Pressures to decrease inspection cost may force inspectors to adopt higher scanning rates, possibly at the expense of reliable defect detection.
Some typical defects which can interfere with the light transmission characteristics of the mask or the electrical properties of the metallization pattern and which can affect final circuit yields are pinholes, protrusions, scratches, and unetched film.
One possible solution to the inspection reliability versus cost problem is automation. However, complete automation of inspection is considered extremely difficult to achieve both technically and economically. The development of more limited automatic techniques, which reliably detect and display all mask defects which might adversely affect the performance of a circuit fabricated by the use of such a mask, is considered much more realistic. Such a semiautomatic scheme would relieve the inspector of the mechanical aspects of inspection and free him to concentrate on the decision function, as to the effects on circuit performance if a detected defect is not corrected provided that an efficient interface is established between the inspector and the inspection machine.
Some semiautomatic optical comparison systems have been described in the literature and are in commercial use. These systems typically utilize at least two synchronously scanning focused light sources to illuminate the patterned masks or workpieces which are placed in optically equivalent positions. A point-by-point comparison of the readings provided by the scanning sources indicates any relative differences between the two patterns. Unfortunately, pattern dimensions vary within some acceptable range from workpiece to workpiece. For some fine line patterns, these dimensional variations result in acceptable pattern aberrations which are of the same approximate size as potentially critical defects. These defects can adversely affect the circuits formed by the use of the inspected masks.
Accordingly, it is one object of the present invention to separate acceptable pattern aberrations from true mask imperfections, the latter being potentially capable of causing failures in circuits fabricated by the use of uninspected masks.
Another object is to achieve a more uniform quality in the circuits fabricated by the use of inspected masks by more accurately and more consistently controlling the decision threshold between acceptable and unacceptable mask patterns.
A further object of the present invention is to reduce the cost of making comparative inspections by decreasing operator interaction with the optical scanning equipment.